Since a lens in a headlamp for an automobile requires transparence, great strength and heat resistance, it is generally formed of a PC. On the other hand, a lamp body in the headlamp generally has excellent heat resistance and chemical resistance and is formed of a tough and inexpensive polypropylene resin (hereinafter referred to as a PP).
Since the PP does not have a polarity, however, it has a poor adhesive property or welding property to other resin products (for example, a lens formed of the PC). For this reason, it is hard to bond or thermally weld the lamp body formed of the PP and the lens formed of the PC. Therefore, there has conventionally been employed, as a method of assembling and fixing both of them, a hot melt method of forming a seal groove capable of engaging a seal leg on a lens side with the peripheral edge portion of the opening portion of a front surface in a lamp body, inserting the seal leg in the seal groove loading a hot melt and fixing and holding an engaging portion between both of them through mechanical fastening means such as a clip, a concavo-convex lance engagement or a screw, thereby carrying out sealing.